First lady wears clothes of satin — they were magnificent in the film (5)
I believe the answer is:
seven
'they were magnificent in the film' is the definition.
(reference to The Magnificent Seven)
'first lady wears clothes of satin' is the wordplay.
'first lady' becomes 'eve' (I've seen this before).
'wears' is an insertion indicator.
'clothes of' suggests removing the centre.
'satin' with its centre removed is 'sn'.
'eve' going within 'sn' is 'SEVEN'.
(Other definitions for seven that I've seen before include "Figure of dwarfs?" , "Wonders of the World - Deadly Sins" , "Number of sisters in Sussex" , "Theme of exotic dance" , "Cardinal" .)